The control of cellular transformation by virus and cell genes will be examined using: a) a morphological revertant of MSV-transformed normal rat kidney (NRK) cells which is morphologically normal but retains growth properties of transformed cells in vitro and is tumorigenic in vivo; b) NRK cells infected with a cold-sensitive transformation mutant of MSV in which the morphological and growth parameters of transformation are conditionally dissociated; and c) chemical carcinogen-transformed, MSV-transformed, and a revertant subclone of MSV-transformed human cells. Using these well characterized cell systems we propose to: 1) Identify virus-specific transforming proteins that are essential to in vivo tumorigenicity and distinguish them from proteins involved in morphological transformation of cells. Transforming gene products will be detected in radiolabeled cellular extracts by immunoprecipitation using antiserum from rats and rabbits bearing tumors induced by MSV-transformed NRK and rabbit cells, respectively. 2) Examine alterations in host cell polypeptides that accompany the morphological or growth phenotype of transformaton or which may restrict or modify expression of transforming proteins resulting in reversion of the transformed cell. Whole cell or plasma membrane fractions will be analyzed by one- and two-dimensional gel electrophoresis. The dissociation of the morphological from the growth phenotype of transformation suggests that the MSV src gene product has more than one primary intracellular target. Temperature-sensitive (ts) mutants of MSV have been useful in elucidating mechanisms of transformation and have identified morphological and cell surface changes associated with transformation. Our objective is to isolate ts growth mutants of MSV based on the ability of MSV-transformed cells to grow in semi-solid medium and selection with cytosine arabinoside. Mutants will be characterized for growth and morphological parameters of transformation at permissive and non-permissive temperatures.